It is proposed to continue our studies of the mechanisms whereby retinoids can inhibit carcinogen induced neoplastic transformation. Using model in vitro systems in which chemical and physical carcinogens induce neplastic change and in which retinoids are active in preventing that change, we propose to conduct following investigations. These studies are directed towards answering two major questions which are limiting the development of retinoids as effective cancer preventive agents in the clinic: 1) how to retinoids work at the cellular and biochemical level; 2) can the potential for retinoids to act as tumor promoters be avoided? Based on previously funded work and on recent developments on cellular control mechanisms, we hypothesize that retinoids act by modulating cellular phosphorylation/dephosphorylation reactions and propose a series of experiments to test this hypothesis. At the biological level, cultures will be treated with known modulators of protein phosphorylation and their effects alone and in conjunction with retinoids on the neoplastic transformation of cells monitored. At the biochemical level the phosphorylation state of cellular proteins will be monitored. In addition we will conduct extensive studies of phosphorylation of recently described residual proteins, possibly associated with the cytoskeleton, and of the composition and phosphorylation of H1 histones. Altered phosphorylation of both these substrates could induce the pleiotropic effects of retinoids. Phamacological studies will also be conducted to determine if the tumor promoting activity of retinoic acid can be explained by its inhibitory effects on intracellular communication and if this effect can be avoided by the use of retinol-like compounds which do not possess this property. These studies should aid in the rationale development of cancer chemoprevention strategies.